1. Field of the Invention
The present invention generally relates to an electrophotography type image forming apparatus and, more particularly, to an image forming apparatus which can be suitably applied to various types of color copying machines such as a multi-color electrophotography copying apparatus comprising a plurality of developers; a recording apparatus constituting an output unit of a facsimile machine, a computer, or the like; a color printer, and the like.
2. Related Background Art
FIG. 14 is a schematic side view of a conventional multi-color electrophotography copying apparatus. Referring to FIG. 14, after the surface of a photosensitive drum 51 is charged by a primary charger 52, an optical image is exposed on the surface of the drum 51, thus forming an electrostatic latent image. The electrostatic latent image is developed by a toner, and the toner image on the photosensitive drum is transferred onto a transfer material P carried on a transfer drum 55 by a transfer charger 55b. After the transfer operation, the transfer material is subjected to electricity removal by electricity removal chargers 55d and 55e.
Although the multi-color electrophotography copying apparatus with the above-mentioned arrangement operates very well, the present inventors found from the results of their studies and experiments that a problem was posed in the transfer process, especially when a polyvinylidene chloride resin film or the like is used as a transfer material carrier sheet 501 of the transfer drum 55, and a transfer paper sheet is used as the transfer material P, or especially when the humidity is high.
FIG. 16 is an explanatory view showing the state of electric charges on an end portion, in particular, a trailing end portion Pa, of the transfer material P on the transfer unit of a transfer device 55A. Although a toner image of one color has already been transferred onto the transfer material P on the transfer drum 55, the transfer material P is kept wound around the transfer drum 55 without being separated therefrom, and is rotated together with the transfer drum 55 so as to transfer a toner image of the next color thereon. The polarity of a transfer voltage to be supplied to the transfer charger 55b is set to be plus (positive) when, for example, an electrostatic latent image is formed by minus (negative) electric charges, and the toner particles of a developing agent in each developing device are charged to have minus polarity so as to reverse and develop the latent image.
The present inventors found from the results of their studies and experiments that when a polyvinylidene chloride resin film was used as the transfer material carrier sheet 501 and a transfer paper sheet was used as the transfer material P, since the volume resistance of the polyvinylidene chloride resin film was 10.sup.13 .OMEGA.cm and the volume resistance of the transfer paper sheet was 10.sup.9 .OMEGA.cm (at high humidity) to 10.sup.12 .OMEGA.cm (at low humidity), plus electric charges from the transfer charger 55b were injected into the transfer material P via the transfer material carrier sheet 501 and were accumulated in the surface area of the transfer material P.
Also, the present inventors learned the following fact. That is, the plus electric charges accumulated in the surface area of the transfer material P generated a high electric field between themselves and the surface of the photosensitive drum 51, and caused peeling discharge when the transfer material P was separated from the photosensitive drum 51, as shown in FIG. 17. Minus electric charges generated in the air due to the peeling discharge moved onto the transfer material P while being attracted by the plus electric charges of the transfer material P, but plus electric charges in the air moved onto the photosensitive drum 51 which was charged to have minus electric charges, thus damaging the photosensitive drum 51, i.e., generating a memory on the drum 51.
The memory decreases the primary charge amount on the photosensitive drum 51 by the primary charger 52 in a stripe shape in the axial direction of the photosensitive drum 51, and consequently disables primary charging of the photosensitive drum 51, thus causing a considerable image defect. The above-mentioned memory is mainly generated in correspondence with the operation of the transfer charger 55b. As shown in FIG. 16, the plus electric charges are easily accumulated especially on the end portion Pa of the transfer material P, strongly generate a memory consequently, and form strong stripe-shaped image nonuniformity in the axial direction of the photosensitive drum 51.
For the purpose of eliminating the memory, another charger (not shown) is arranged at the output side of the transfer charger 55b to perform electricity removal of the memory area on the photosensitive drum 51 after the transfer process, or the photosensitive drum 51 is subjected to electricity removal exposure and primary charge processes to pre-charge the drum 51 to have the same polarity as that obtained by the primary charge process so as to sufficiently eliminate the memory area. However, a sufficient effect cannot be obtained by only electricity removal. On the other hand, when the drum 51 is charged to have the same polarity as that obtained by the primary charge process, the elimination effect of the memory area is observed. However, in this case, since residual toner particles on the photosensitive drum 51 are also charged, they often cause a cleaning error.
When the photosensitive drum 51 is pre-charged to have the same polarity as that obtained by the primary charge process, the drum 51 must be charged to have a considerably high potential so as to sufficiently eliminate the memory generated on the end portion Pa of the transfer material P. In such a case, an image defect is often generated due to dielectric breakdown of the photosensitive drum 51. Furthermore, an extra space is required. More specifically, since a conventional transfer operation is performed in a state wherein the photosensitive drum 51 contacts the transfer material P, there is no effective means for eliminating the memory generated on the photosensitive drum 51 and, more particularly, the strong memory generated on the area corresponding to the end portion Pa of the transfer material without causing a cleaning error, dielectric breakdown of the photosensitive drum 51, and the like and without requiring an extra space.